1. Field of the Invention
The present invention relates to a microresonator.
2. Discussion of the Related Art
FIG. 1 is a perspective view of a known microresonator. This microresonator is described in patent WO-02/17482 entitled “Micromechanical resonator device and micromechanical device utilizing same”. This microresonator is formed above a substrate 1. It comprises a “mushroom”-shaped resonant element 2 formed of a cylindrical head resting on a foot attached to the substrate. Two metal pads 3 and 4 are placed on each side of resonant element 2. The vertical wall of each of pads 3 and 4 located opposite to the wall of the cylindrical head is curved and surrounds the cylindrical head. The distance between resonant element 2 and pads 3 and 4 is very short, on the order of from a few tens of nanometers to one micrometer.
When an A.C. voltage is applied between resonant element 2 and pads 3 and 4, resonant element 2 tends to deform by expanding or retracting. When resonant element 2 starts resonating, the resonant element expands and retracts at the resonance frequency. Pads 3 and 4 and resonant element 2 are then equivalent to a capacitor having its capacitance varying at the resonance frequency.
The previously-described microresonator can be used in various fashions. An example of use as a filter is described hereafter. As shown in FIG. 1, resonant element 2 is connected to a bias voltage Vpol via a coil L and connected to a resonance detection circuit via a capacitor C. The detection circuit is represented by a charge resistor Rc placed between capacitor C and the ground. Pads 3 and 4 receive an input voltage ve. When voltage ve comprises a D.C. component and an A.C. component varying at the resonance frequency, a current is varying at the resonance frequency is provided to the detection circuit. When voltage ve varies at a frequency different from the resonance frequency, the variation of current is provided to the detection circuit is substantially zero.
A method for manufacturing the microresonator shown in FIG. 1 is described in the above-mentioned patent. This method is complex and comprises a great number of steps. The four big phases of this method are the following.
In a first phase, illustrated in FIG. 2A, a hollow insulating portion 11 formed for example of silicon oxide is formed on a substrate 10.
In a second phase, illustrated in FIG. 2B, a silicon element 12 having the shape of a mushroom comprising a foot placed in the hole of insulating portion 11 and a cylindrical head resting on insulating portion 11 is formed.
In a third phase, illustrated in FIG. 2C, a thin silicon oxide layer 13 is formed around resonant element 12 and on the free portions of insulating portion 11. Conductive pads 14 and 15 are then formed on substrate 10 on each side of element 12. The conductive pads are in contact with silicon oxide layer 13.
In a fourth phase, illustrated in FIG. 2D, silicon oxide layer 13 and insulating portion 11 are eliminated. A resonator such as illustrated in FIG. 1 is then obtained.
Apart from its complexity, the previously-described method has the disadvantage of forming a resonant element having a polycrystalline structure. Indeed, resonant element 12 is obtained by silicon deposition on a silicon oxide layer, which results in forming polysilicon. The polycrystalline structure of the resonant element is a disadvantage since this causes mechanical weaknesses. Further, the resonance frequency of a polysilicon resonant element may vary from one resonant element to another.